The electrical windings (of the rotor or stator) of electrical machines working at high voltage, e.g. electric generators or motors, in particular slip-ring rotor motors, have end windings at the ends in which the winding bars protruding from the slots of the laminated cores are conductively connected to one another in pairs. The electrical connection is frequently made by means of lugs which are soldered to the conductor ends of the winding bars from which the insulation has been removed. The connecting points are then electrically insulated by pushing on special insulating caps. Examples of such end windings are described in publications DE 1 281 015 B, DE 42 37 079 A1, EP 1 821 389 A1 or U.S. Pat. No. 4,309,636 A.
Insulating caps for end windings of electrical machines working at high voltage which are at high-voltage potential cannot be earthed on the outside.
Furthermore, a gradient material can be provided on the inside of insulating caps (see, for example, WO 2004038735 A1) in order to reduce the electrical field strength in the interior of the insulating cap. The outside of insulating caps of this type is still at high voltage however, so that the external surface cannot be earthed.